Gas purification, more specifically air prepurification, represents a class of adsorption separation processes where multiple adsorbents can be applied to improve process performance. The operation of cryogenic air separation plants requires large quantities of pretreated air. To prevent freezing and plugging of the primary heat exchanger, the concentration in the pretreated air of contaminants or impurities such as CO2 and H2O are required to be lowered to less than 1 ppm. In addition, the concentration of light hydrocarbons such as acetylene which have a low solubility in cryogenic liquids must be kept very low, typically less than 1 ppb, to prevent accumulation within the cryogenic distillation system. Nitrogen oxides (e.g., N2O) also need to be removed to the sub ppm level.
Removal of contaminants or impurities can usually be accomplished by an adsorption process employing two or more vessels containing one or more adsorbents selective towards the impurities. When an adsorption bed is saturated with impurities, the bed needs to be regenerated by either one or a combination of two different general methods: pressure swing adsorption (PSA), during which a change in pressure is utilized to regenerate the sorbent, or temperature swing adsorption (TSA), during which the impurities are desorbed by using a thermal driving force such as a heated purge gas. The TSA process may also optionally superimpose a pressure swing to enhance its regeneration capability and reduce its purge requirement. The TSA process usually requires a much lower amount of purge flow relative to the PSA process and affords a longer cycle time, typically in the range of about 4-10 hours. On the other hand, the PSA process typically requires a greater amount of purge flow and has a much shorter cycle time, on the order of 10-50 minutes. The PSA process, however, can operate with ambient feed temperatures contrary to the TSA process, which typically needs a feed cooled to sub-ambient temperature by means of a refrigeration system. Moreover, there is no requirement for regeneration heat energy in PSA as opposed to TSA.
When there is sufficient waste nitrogen available from a cryogenic air separation plant, the nitrogen can be used as the purge flow gas as it typically contains no impurities and would otherwise be vented. Accordingly, when such nitrogen is available, PSA is therefore usually a preferred option for air prepurification due to its simplicity, lower capital cost as well as lower operating cost.
Notwithstanding the advantages of the PSA process compared to the TSA process, PSA processes have been limited in that the adsorbents are typically not completely regenerated at the completion of the purge step. Consequently, the bed dynamic capacity is less than it would be for a TSA process. As a result, the PSA process is typically run for short cycle times which thus necessitates that the bed(s) undergo blowdown and repressurization at fairly frequent intervals. During the blowdown step, there is a noticeable loss of air trapped within the void spaces of the vessel(s) and piping as well as the air adsorbed on the adsorbents therein. This collective air loss, referred to by various terms such as blowdown loss, vent loss or bed switch loss, can represent a significant energy waste as the air is compressed but not utilized for air separation downstream of the prepurifier. Reducing the blowdown loss can provide significant operating cost savings in terms of reduced compression power.
There are other disadvantages associated with frequent bed switches in a PSA cycle. For example, in a dual bed PSA process, the repressurization phase can cause upsets in the flow of purified air to the cryogenic distillation columns downstream of the prepurifier. Such frequent flow fluctuations can disturb the dynamics of the distillation process, thus resulting in lower efficiency for air separation in addition to causing a variation in the product purity.
Most prior art techniques to reduce or minimize the blowdown loss in a PSA process have focused on the reduction of the co-adsorption of the bulk components of air, namely O2 and N2, on a per cycle basis. Such techniques prompt the selection of an adsorbent configuration with a larger proportion of a weak adsorbent such as activated alumina which has very low capacity for O2 and N2, and a relatively smaller proportion of the stronger adsorbent, such as a molecular sieve.
An alternative approach to lower the power requirement of the PSA process is to reduce the frequency of the blowdown or bed switch loss mentioned above. This can be accomplished by extending the cycle time for which the bed is kept online prior to being switched to regeneration. Because the adsorbents and the bed configurations described in the prior art typically afford fairly modest dynamic capacities for impurity removal, an increase in cycle time would require either reducing the feed flow significantly at a fixed bed size, or require a significant increase in the bed size at a fixed feed flow rate. Both of these options can have adverse consequences on the capital and operating costs of the PSA prepurification process.
K. Chihara and M. Suzuki, “Simulation of Nonisothermal Pressure Swing Adsorption,” Journal of Chemical Engineering of Japan, Vol. 16, No. 1, pg. 53-61 (1983) describe a computer simulation study of a non-isothermal PSA case study involving the drying of air using a single layer bed composed of either activated alumina or silica gel. An optimization of various process parameters such as bed length, cycle time and purge to feed ratio was presented. It is suggested from this work that an increase in adsorption cycle time would either require a longer bed length or a higher purge to feed ratio to maintain the product purity at a desired level.
German Patent Application DE 3,045,451 A1 (1981) describes a PSA process in which air is passed through a first stage having 13× zeolite to remove CO2 and H2O in their high concentration zones, and then through a second stage having activated alumina to remove the remaining CO2 and H2O in their low concentration zones.
U.S. Pat. No. 4,711,645 to Kumar proposes a PSA process with improved energy savings relative to conventional TSA processes. The PSA process includes feeding air through an initial layer of alumina for H2O removal followed by a bed of zeolite for CO2 and residual H2O removal. The lower heat of adsorption of H2O in alumina compared to that of water in zeolite reportedly results in a smaller temperature rise and improves the bed capacity for CO2 removal in the downstream layer of zeolite.
U.S. Pat. No. 5,232,474 to Jain relates to a PSA process in which an alumina layer is reportedly designed to remove at least 75 mole percent of the CO2 present in a feed stream containing at least 250 ppm of CO2. The feed may optionally be passed through a second adsorption zone containing a zeolite such as 13× to remove residual CO2 and hydrocarbons. In such layered configurations, the alumina occupies more than 80% of the total bed volume.
U.S. Pat. No. 5,769,928 to Leavitt discusses a PSA bed composed of at least two discrete layers of adsorbents, at least one of the adsorbents being comparatively strong and at least another of the adsorbents being comparatively weak with respect to the adsorption of water and other contaminants. More specifically, the patent relates to the use of a comparatively weaker adsorbent such as activated alumina, followed by a stronger adsorbent such as NaY. This configuration is said to ensure a consistent breakthrough of CO2 ahead of the C2H2 front, providing improved plant safety.
U.S. Pat. No. 5,779,767 to Golden et al. relates to a mixture of adsorbent composed of activated alumina (or an alkali-modified alumina) and a zeolite without maintaining the two adsorbents in separate beds or layers for the removal of various air impurities. Such a bed design reportedly has a high working capacity for CO2 to reduce bed size. In addition, the adsorbents are said to have high reversible capacity for acetylene, water and nitrogen oxides.
The use of an activated alumina and zeolite composite or a homogeneous mixture formed by blending beads of activated alumina and zeolite for the removal of CO2 from feed streams is also disclosed in Jain, et al., EP 0 904 825 A2. H2O in the feed may be removed in the mixed alumina-zeolite layer itself or by using a preliminary layer containing a desiccant such as activated alumina or silica gel.
Ackley et al., in U.S. Pat. No. 6,027,548, propose a PSA prepurifier bed composed of a mixture or a composite of at least two adsorbents, one of which is comparatively strong (e.g., NaY or NaX) and the other which is comparatively weak (e.g., activated alumina). Such a bed configuration is said to preferentially adsorb acetylene or C3-C8 hydrocarbons over CO2 and is self-cleaning with respect to these contaminants at a lower purge than that required by 13× zeolite. A preferred embodiment is to utilize activated alumina near the feed end and the mixed adsorbent near the product end of the bed.
The removal of CO2 and H2O from air using a layered bed of y-alumina and 13× zeolite using numerical computer simulations is discussed in Rege et al., “Air-Prepurification by Pressure Swing Adsorption Using Single/Layered Beds,” Chemical Engineering Science, Vol. 56 No. 8, pg. 2745-2759 (2001). At certain fixed process conditions such as constant bed length, purge to feed ratio, feed flow and cycle time, the relative proportion of alumina and 13× zeolite layer heights in the bed were varied to reportedly optimize the design. The authors concluded that a minimum impurity concentration results when the ratio of alumina to the zeolite is 7:3.
Given the growing cost of energy worldwide, there is an increasing need to reduce power and increase the operational efficiency of the PSA prepurification process. In view of the teachings of the prior art, it would therefore be desirable to provide an adsorbent zone configuration suitable for use in a PSA prepurifier that allows for extension in PSA cycle times and that can lower blowdown loss and reduce operating costs associated therewith.